1. Field of the Invention
The present invention relates to an optical disk reading device, and in particular, to a method for improving the balanced position of the balls in an automatic ball balancing system. In particular, the present invention relates to a method which reduces the movement resistance on the surface of a track in the gravity direction of the balls to change the rotation speed of the rotor and the system phase angle of the balls when reaching the rotation speed, so as to improve the balancing effect of the automatic ball balancing system.
2. Description of the Prior Art
General optical disk reading devices, such as CD-ROM, DVD-ROM, CD-RW, DVD-RAM, and other optical data reproducing or recording devices, have been widely used in multimedia computer systems and have become an important component among the peripheral devices of computer systems.
The reading speed of an optical disk drive has increased significantly with the development of optical data storage medium technology. At present, most mainstream optical disk devices have a rotation speed of the spindle motor that is higher than 10000 RPM.
When a spindle motor rotates at high speeds, a centrifugal deviation force generated by unbalance of the disk is increased, leading to vibration, noise, and other problems. In the practical application of an optical disk drive, excessive vibration will cause instability in the reading ability of the optical head. As a result, the optical disk drive cannot effectively read data at the highest rotation speed. In addition, the noise generated when the optical disk drive rotates at high speed can bother and disturb the user of the optical disk drive. Consequently, how to effectively suppress vibration so that an optical disk drive can read data from the optical disk correctly and smoothly at the highest rotation speed is a problem that has yet to be effectively addressed.
Conventional optical disk drives are provided with an automatic ball balancing system in order to reduce the vibration of an optical disk drive during high-speed rotation caused by unbalance of the rotor of the rotation mechanism. The automatic ball balancing system is typically provided above or below the spindle motor. The automatic ball balancing system is effective in reducing vibration because it uses a method of adding a balancing mass to directly reduce the unbalance of the vibration.
The theory for the balls of the above-mentioned automatic ball balancing system to reach the desired balanced positions is based on the theory of rotor dynamics. FIGS. 1A–1C illustrate three possible conditions for the balls 2 in the automatic ball balancing system. First, when the stable rotation speed of the spindle motor is lower than the unstable critical rotation speed (called natural frequency of the suspending system), the unbalance amount of the balls 2 and the imbalance center of mass of the disk 3 of the system are in the same phase state (as shown in FIG. 1A). Second, when the stable rotation speed of the spindle motor is equal to the unstable critical rotation speed, there is a phase difference of 90° between the unbalance amount of the balls 2 and the imbalance center of mass of the disk 3 of the system (as shown in FIG. 1B). The numeral 4 in FIG. 1B illustrates the position of the balls 2 during this condition. Third, when the stable rotation speed of the spindle motor is higher than the unstable critical rotation speed, there is a phase difference of 180° between the unbalance amount of the balls 2 and the imbalance center of mass of the disk 3 of the system (as shown in FIG. 1C). Again, the numeral 4 in FIG. 1C illustrates the position of the balls 2 during this condition.
However, when the balancing balls 4 rotate and slide along the tracks within the rotor, the balancing balls 4 typically experience a movement resistance in the form of a frictional force opposite to the their motion. This movement resistance may be caused by the degree of roundness, concentric degree, and surface roughness of the balls 2. As a result, when the rotor (or spindle motor) reaches a stable rotation speed, the balls 2 cannot reach the system phase angle for the aforementioned stable rotation speed (that is lower than the unstable critical rotation speed). This opposite frictional force results in a phase angle delay of the balancing balls 4. The greater the frictional force, the greater the phase angle delay of the balancing balls 4. FIGS. 1A–1C describe the ideal result of the ball balancing system. As shown in FIG. 1C, when the stable rotational speed of the spindle motor is larger than the unstable critical rational speed, the phase angle difference is equal to 180 degrees, and vibration due to rotation will be suppressed significantly. On the other hand, if the phase angle difference is less than 180 degrees due to the frictional force, then the vibration cannot be suppressed significantly. Thus, if the phase angle difference between the balls 2 and the imbalance center of mass of the disk 3 is an integral times of 180 degrees, than the vibration will be reduced significantly. This will allow the rotational speed of rotor to be accelerated to greater speeds even when the rotational speed is larger than the unstable rotational speed.
FIG. 7 illustrates the phase angle delay between the balls 2 and the imbalance center of the disk 3.